Ion micro-distribution in varying aged leaves in salt-treated cucumber seedlings

Na⁺ distribution is one of the most important strategies for plant resistance to salt stress. The way of Na⁺ compartmentation in different aged leaves has been controversial, especially at the cell and sub-cellular level. The roles that Na⁺ and K⁺/Na⁺ play the key role in photosynthesis need to be further verified. In this study, using two cucumber cultivars Cucumis sativus L. cv. zhongnong 8 (ZN8, relatively salt tolerant) and Cucumis sativus L.cv. Jinchun 4 (JC4, salt sensitive) as experiment material, we analyzed the mode of ion compartmentation of Na⁺ in organelles in different aged leaves and determined which factors (the organelles' Na⁺ or K⁺/Na⁺) affect leaf photosynthesis, using high-pressure freezing and freeze-substitution, Ultrathin sectioning technique and X-ray. The main results: 1. The sub-cellular trends of Na⁺ accumulation was cell wall > vacuole > cytoplasm > chloroplasts; 2. The Na⁺ accumulation in cytoplasm and chloroplasts was similar in different aged leaves and in seedlings of different salt tolerance cultivars; 3. The K⁺/Na⁺ ratio is the main factor that affects the photosynthesis of the same aged leaves in our experiment. A weak capacity for ion compartmentation may be an important reason leading to salt sensitivity.